1. Field of the Invention
The present invention relates to a sound reproducing apparatus capable of reproducing sound of excellent quality inside of a motor vehicle, for instance.
2. Background Art
FIG. 10 is a system diagram illustrating a conventional sound reproducing apparatus. In FIG. 10, phase shifters 102 and 105, first power amplifier 103 and second power amplifier 106 are connected individually with music signal source 101. An output of first power amplifier 103 is connected to speaker 104 mounted to a left door of motor vehicle 112, and an output of second power amplifier 106 is connected to speaker 107 mounted to a right door of vehicle 112, so as to enable reproduction of music inside the vehicle. In this instance, phase shifters 102 and 105 operate in a manner to suppress standing waves generated by the right and the left walls in the vehicle to reduce an influence of the standing waves, which impedes an auditory image localization in a given position, thereby enabling a listener to enjoy excellent reproduction of music in the given position inside the vehicle.
Japanese Patent Unexamined Publication, No. 2003-47097 is one of the known examples of the prior art documents relating to the invention of this patent application.
It is possible to reduce the influence of standing waves generated by the right and the left walls in the vehicle, and to reproduce music well by using the conventional system described above.
FIG. 11 is a system diagram for use in detailing a drawback of the conventional art. In a speaker layout shown in FIG. 11, in which center speaker 111 is disposed at the front center of the vehicle interior in addition to right and left speakers 104 and 107, there occurs a phase interference between center speaker 111 and right and left speakers 104 and 107, which impairs good music reproduction.
Referring to FIG. 11, description is provided of the fact that the phase interference between right and left speakers 104 and 107 and center speaker 111 cannot be reduced even if an existing technique of phase control is used. In FIG. 11, symbol d1 indicates a distance between left speaker 104 and listening point P1, and symbol d2 indicates a distance between center speaker 111 and listening point P1. Symbol d3 indicates a distance between center speaker 111 and listening point P2, and symbol d4 indicates a distance between right speaker 107 and listening point P2. It is assumed here in order to simplify the explanation that these distances have a relation of d1≈d2≈d3≈d4. In many cases, the above relational formula applies to speaker layouts of the ordinary motor vehicles. In addition, a center frequency of phase reversion of phase shifter 102 is denoted as x, and a center frequency of phase reversion of another phase shifter 105 is denoted as y.
Under the conditions stated above, however, a phase interference occurs at listening point P1 since an output of left speaker 104 is in an opposite phase with respect to an output of center speaker 111 due to the reversion of phase by phase shifter 102 at a frequency corresponding to the center frequency x of phase reversion. Even if another phase shifter 108 of the center frequency x of phase reversion is added to a circuit of center speaker 111 by using the existing phase control technique, the phase interfere still occurs at listening point P2, since the output of center speaker 111 becomes opposite in phase to that of right speaker 107.